Can't get simple light path nod examples to work
I'm not sure what tutorials you've been watching, but this setup should work for you. Nothing special - like your attempt, the floor has a white Diffuse BSDF for it's material, and the material for the cube is shown in the graph. If this doesn't work, please let me know (ideally with a screenshot) what you're seeing instead:
This first image uses the Is Camera Ray output of the light path node to give a solid red shadow with no bounced light glow:

This setup uses the Is Shadow Ray output of the light path node, and gives a red shadow, with green bounced lighting:

Finally, this setup uses both Camera and Shadow rays, and gives a red shadow with red bounced lighting as well: